Spin-unrestricted Density Functional Theory Research Articles

A Comparative Density Functional Theory Study of Hydrogen Storage in Cellulose and Chitosan Functionalized by Transition Metals (Ti, Mg, and Nb).

The focus of this work is hydrogen storage in pristine cellulose, chitosan, and cellulose. Chitosan doped with magnesium, titanium, and niobium is analyzed using spin unrestricted plane-wave density functional theory implemented in the Dmol3 module. The results of this study demonstrate that hydrogen interaction with pure cellulose and chitosan occurred in the gas phase, with an adsorption energy of Eb = 0.095 eV and 0.090 eV for cellulose and chitosan, respectively. Additionally, their chemical stability was determined as Eb= 4.63 eV and Eb = 4.720 eV for pure cellulose and chitosan, respectively, by evaluating their band gap. Furthermore, the presence of magnesium, titanium, and niobium on cellulose and chitosan implied the transfer of an electron from metal to cellulose and chitosan. Moreover, our calculations predict that cellulose doped with niobium is the most favorable medium where 6H2 molecules are stored compared with molecules stored in niobium-doped chitosan with Tmax = 818 K to release all H2 molecules. Furthermore, our findings showed that titanium-doped cellulose has a storage capacity of five H2 molecules, compared to a storage capacity of four H2 molecules in titanium-doped chitosan. However, magnesium-doped cellulose and chitosan have insufficient hydrogen storage capacity, with only two H2 molecules physisorbed in the gas phase. These results suggest that niobium-doped cellulose and chitosan may play a crucial role in the search for efficient and inexpensive hydrogen storage media.

Spin Alternation Rule in USCF for Through-Bond Magnetic Coupling─A New Look: Why and When Does It Arise and How To See It.

This work takes a new look at the spin alternation rule in unrestricted self-consistent-field (USCF) calculations in terms of structural characteristics such as periodicity, impurity location, and Coulomb exchange. For clarity, the systems considered are biradicals produced from linear conjugated hydrocarbons. Both site-parametrized Hamiltonian models for theoretical analysis and spin unrestricted density functional theory (DFT) calculations are used. Theoretical analysis leads to the following conclusions: (1) The diradical state is an excited state of a linear chain of N conjugated carbon atoms (when N is about ≤ 10). Spin alternation is a consequence of the (truncated) periodic symmetry combined with filling each closed-shell pi orbital with two electrons and each singly occupied molecular orbital (SOMO) with one electron. Spin polarization is evident in triplet (T) and broken symmetry (BS) solutions for an odd N and only in the T solution for an even N. Spin alternation is visible in the BS for an odd N and always remains muted in the calculated T. (2) For a doped chain with two radical centers, spin alternation is generally visible in the BS for an odd N. The sign of spin population on the radical centers in the BS indicates the stable spin. For radical centers separated by an odd (even) number of pz electrons, spin alternation favors T (S) state with FM (AFM) interaction. Spin oscillation remains less transparent for an even N without exchange. (3) In an unrestricted treatment with exchange, spin alternation becomes observable. Without SCF iterations, the more stable state can be identified from a clear spin oscillation in the BS. An irregular oscillation indicates a possible singlet ground state. These observations are supported by density functional calculations using the B3LYP functional and the 6-311+g(d,p) basis set on linear decapentaene diradicals with nitronyl nitroxide moieties substituted on two sets of conjugated atoms, (3,9) and (3,10). Because of the SCF procedure, one finds spin alternation in the T (BS) solution and erratic oscillation in the BS (T) solution of the 3,9 (3,10) diradical in respective equilibrium geometries. The ground state is T (S). DFT adiabatic coupling constants, SOMO energies, spin population plots, and SOMO lobe diagrams compare well with molecular electronic characteristics from theoretical analysis using Hamiltonian parameters.

Open Shells in Endohedral Clusters: Structure and Bonding in the [Fe2@Ge16]4- Anion and Comparison to Isostructural [Co2@Ge16]4.

The anionic cluster [Fe2@Ge16]4- has been characterized and shown to be isostructural to the known D2h-symmetric α isomer of the cobalt analogue [Co2@Ge16]4-. Together with the known pair of compounds [Co@Ge10]3- and [Fe@Ge10]3-, the title compound completes a set of four closely related germanium clusters that allow us to explore how the metal-metal and metal-cage interactions evolve as a function of size and of the identity of the metal. The results of spin-unrestricted density functional theory (DFT) and multiconfigurational self-consistent field (MC-SCF) calculations present a consistent picture of the electronic structure where transfer of electron density from the metal to the cage is significant, particularly in the Fe clusters where the exchange stabilization of unpaired spin density is an important driving force.

Spin-projected QM/MM Free Energy Simulations for Oxidation Reaction of Guanine in B-DNA by Singlet Oxygen.

Guanine is the most susceptible base to oxidation damage induced by reactive oxygen species including singlet oxygen (1O2, 1Δg). We clarify whether the first step of guanine oxidation in B−DNA proceeds via either a zwitterionic or a diradical intermediate. The free energy profiles are calculated by means of a combined quantum mechanical and molecular mechanical (QM/MM) method coupled with the adaptive biasing force (ABF) method. To describe the open‐shell electronic structure of 1O2 correctly, the broken‐symmetry spin‐unrestricted density functional theory (BS−UDFT) with an approximate spin projection (AP) correction is applied to the QM region. We find that the effect of spin contamination on the activation and reaction free energies is up to ∼8 kcal mol−1, which is too large to be neglected. The QM(AP−ULC−BLYP)/MM‐based free energy calculations also reveal that the reaction proceeds through a diradical transition state, followed by a conversion to a zwitterionic intermediate. Our computed activation energy of 5.2 kcal mol−1 matches experimentally observed range (0∼6 kcal mol−1).

Sulfur-Doped (Dibenzo)heptazethrene and (Dibenzo)octazethrene Diradicaloids.

We report the synthesis and characterization of sulfur-doped dibenzoheptazethrene (DBHZ-2S) and dibenzooctazethrene (DBOZ-2S) dications. Their neutral precursors 9a/9b were synthesized by acid-mediated intramolecular cyclization reaction of sulfoxides and the dications were prepared by treating 9a/9b with NOSbF6 or AgSbF6, respectively. Both dications displayed open-shell singlet ground state with a moderate diradical character (y0 = 0.44 for DBHZ-2S and 0.66 for DBOZ-2S) and a small singlet-triplet energy gap (ΔES-T = -3.9 kcal/mol for DBHZ-2S and -3.2 kcal/mol for DBOZ-2S). The open-shell singlet diradical nature of DBHZ-2S and DBOZ-2S were supported by electronic absorption spectroscopy, electron spin resonance spectroscopy, variable-temperature NMR studies, together with spin-unrestricted density functional theory calculations. However, synthesis and isolation of the sulfur-doped heptazethrene (HZ-2S) and octazethrene (OZ-2S) dications upon chemical oxidation of their corresponding neutral precursors 5a/5b were not successful owing to their high reactivity.

Understanding the correlation between the electronic structure and catalytic behavior of TiC(001) and TiN(001) surfaces: DFT study

Spin-unrestricted density functional theory (DFT) calculations are performed to explore the electronic structure and water-gas shift (WGS) behavior for the non-polar (001) surface of both titanium carbide (TiC) and titanium nitride (TiN). Previous researches have suggested that the unique electronic structures of these materials can effectively explain their surface adsorption and reactivity. Herein, our DFT calculations demonstrate that TiC(001) surface with occupied nonmetal C-2p states strongly binds CO molecule to the carbon site, and subsequently mediates the WGS along an associative mechanism. In contrast, TiN(001) surface dominated by metal Ti-3d states prefers to adsorb CO molecule at the titanium site, and then mediates WGS reaction through a redox mechanism. This indicates that the surface behavior of TiC(001) and TiN(001) is strongly dependent on their own electronic structure as expected. Therefore, the present studies provide an interesting perspective to understand the correlation between the electronic structure and catalytic behavior of such materials.

Seven Clues to Ligand Noninnocence: The Metallocorrole Paradigm.

Noninnocent ligands do not allow an unambiguous definition of the oxidation state of a coordinated atom. When coordinated, the ligands also cannot be adequately represented by a classic Lewis structure. A noninnocent system thus harbors oxidizing (holes) or reducing equivalents (electrons) that are delocalized over both the ligand and the coordinated atom. To a certain degree, that is true of all complexes, but the phenomenon is arguably most conspicuous in complexes involving ligands with extended π-systems. The electronic structures of such systems have often been mischaracterized, thereby muddying the chemical literature to the detriment of students and newcomers to the field. In recent years, we have investigated the electronic structures of several metallocorrole families, several of which have turned out to be noninnocent. Our goal here, however, is not to present a systematic account of the different classes of metallocorroles, but rather to focus on seven major tools (in a nod to A. G. Cairns-Smith's Seven Clues to the Origin of Life) that led us to recognize noninnocent behavior and subsequently to characterize the phenomenon in depth. (1) The optical probe: For a series of noninnocent meso-triarylcorrole derivatives with different para substituents X, the Soret maxima are typically exquisitely sensitive to the nature of X, red-shifting with increasing electron-donating character of the group. No such substituent sensitivity is observed for the Soret maxima of innocent triarylcorrole derivatives. (2) Quantum chemistry: Spin-unrestricted density functional theory calculations permit a simple and quick visualization of ligand noninnocence in terms of the spin density profile. Even for an S = 0 complex, the broken-symmetry method often affords a spin density profile that, its fictitious character notwithstanding, helps visualize the intramolecular spin couplings. (3) NMR and EPR spectroscopy: In principle, these two techniques afford experimental probes of the electronic spin density. (4) Structure/X-ray crystallography. Ligand noninnocence in metallocorroles is often reflected in small but distinct skeletal bond length alternations in and around the bipyrrole part of the macrocycle. In addition, for Cu and some Ag corroles, ligand noninnocence manifests itself via a strong saddling of the macrocycle. (5) Vibrational spectroscopy. Unsurprisingly, the aforementioned bond length alternations translate to structure-sensitive vibrational marker bands. (6) Electrochemistry. Noninnocent metallocorroles exhibit characteristically high reduction potentials, but caution should be exercised in turning the logic around. A high reduction potential does not necessarily signify a noninnocent metallocorrole; certain high-valent metal centers also undergo metal-centered reduction at quite high potentials. (7) X-ray absorption spectroscopy (XAS). By focusing on a given element, typically the central atom in a coordination complex, X-ray absorption near-edge spectroscopy (XANES) can provide uniquely detailed local information on oxidation and spin states, ligand field strength, and degree of centrosymmetry. For metallocorroles, some of the most clear-cut distinctions between innocent and noninnocent systems have come from the K-edge XANES of Mn and Fe corroles. For researchers faced with a new, potentially noninnocent system, the take-home message is to employ a good majority (i.e., at least four) of the above methods to arrive at a reliable conclusion vis-à-vis noninnocence.

Corrigendum: Open‐Shell Characters, Aromaticities and Third‐Order Nonlinear Optical Properties of Carbon Nanobelts Composed of Five‐ and Six‐Membered Rings

In the original article,1 we reported that electronic structures and optical and magnetic response properties of an open-shell carbon nanobelt (IF-CNB) are evaluated by the spin-unrestricted density functional theory (U-DFT) method using the long-range corrected LC-BLYP exchange correlation functional, namely, at the LC-UBLYP/6-311+G** level. However, only in the calculations of magnetic response properties, we found that the results are obtained by the spin-restricted DFT (R-DFT) solution with <S2>=0. Therefore, the magnetically-induced current (MIC) density maps given in Figures 6 and S6 (in the Supporting Information) as well as the nucleus-independent chemical shift (NICS) values given in Figure S5 for the open-shell IF-CNB are found to be those evaluated at the LC-RBLYP/6-311+G** level. We sincerely apologize for this mistake. In a previous study,2 on the other hand, we compared the MIC density results from the R- and U-DFT solutions for oligoacenes and dicyclopenta-fused oligoacenes by using the gauge-including MIC (GIMIC) method. We found that the diatropic (aromatic) contributions of aromatic (4n+2)π systems with open-shell singlet electronic structures tend to be exaggerated when the R-DFT solution was employed primarily because of its very small energy gaps for the orbital transitions contributing to the diatropic current. Therefore, we have recalculated the MIC density map for IF-CNB at the LC-UBLYP/6-311+G** level of approximation and compared the result with that at the LC-RBLYP/6-311+G** level. The results are shown in Figure 1. Even though strong contributions of diatropic (aromatic) ring current were predicted everywhere around the belt at the LC-RBLYP level (Figure 1a), such strong aromatic contributions of ring current are found to be suppressed at the LC-UBLYP level (Figure 1b). In particular, paratropic (anti-aromatic) contributions are found to be predominant inside the belt whereas diatropic contributions are found to appear outside the belt at the LC-UBLYP level. Such paratropic/diatropic contributions inside/outside the hoop are similar to those of [6]annulene (benzene). However, the amplitudes of integrated MIC values of IF-CNB (1-3 nA/T) are found to be much smaller than that of the closed-shell [6]annulene (11.9 nA/T) evaluated at the LC-RBLYP/6-311+G** level [2]. Figure 2 shows the side-views of the MIC density map and the integrated values of MIC density for each bond calculated at the LC-RBLYP and LC-UBLYP levels. Even though the amplitudes of integrated MIC values are decreased, the net induced ring currents along the belt are predicted to flow clockwise at the LC-UBLYP level. Magnetically induced current (MIC) density vector plots of IF-CNB evaluated at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** (b) levels on the plane perpendicular to the z-axis and 1 Å above the center of mass. Diatropic currents rotate clockwise, while the paratropic ones anticlockwise. The values in the color bar are given in a.u. The current vectors are scaled with a factor of 30 Å a.u.-1, where 1 a.u.=100.63 nA T−1 Å−2. Integrated MIC values [nA/T] of each bond with along with MIC maps for IF-CNB at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** (b) levels. Positive and negative values represent clockwise (diatropic) and counter-clockwise (paratropic) contributions of ring current, respectively. Spatial integration of MIC densities was performed on the bisector plane of the bond with a width of 2.0 Å. The length of the plane is 8.0 Å from the center of the mass of the belt. Here, the current vectors are scaled with a factor of 20 Å a.u.−1. Figure 3 shows the results of NICS(0) values on the five- and six-membered rings, which characterize aromatic nature on each ring. In both R- and U-DFT cases, positive (negative) NICS values at the center of five- (six-) membered ring are obtained, whereas, the U-DFT results are found to exhibit more antiaromatic nature than the R-DFT results. This feature may be associated with the emergence of the odd electron density in the U-DFT solution. NICS(0) values [ppm] at the centers of five- and six-membered rings calculated at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** levels. NICS(0) values of symmetry-independent positions are only shown. In summary, the MIC density map of IF-CNB in Figure 6 in the original article should be replaced by Figure 1b, the correct integrated MIC values corresponding to Figure S6 in the Supporting Information are shown in Figure 2b, and the NICS(0) values in Figure S5 (Supporting Information) are presented in Figure 3b. Judging from the results at the LC-UBLYP level, IF-CNB is expected to show not so strong, but fairly weak aromatic nature over the belt in the open-shell singlet state.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water–gas shift performance: DFT study

The water–gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (PtML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the PtML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make PtML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968–74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Theoretical Study on Second Hyperpolarizabilities of Intramolecular Pancake-Bonded Diradicaloids with Helical Scaffolds

Using the spin-unrestricted density functional theory method, we have investigated a novel class of open-shell singlet fused-ring molecules with helical scaffolds toward the development of highly efficient third-order nonlinear optical (NLO) systems. It turns out that the helical molecules possess pancake bonding with intermediate diradical character and thereby exhibit large enhancement of orientationally-averaged second hyperpolarizability γs values (third-order NLO properties at the molecular scale) as compared to their closed-shell analogues. It is also found that the asymmetric charge distribution between the cofacial radical sites located on both the terminal rings of the helical scaffold leads to further enhancement of the γ value in the intermediate open-shell character region as compared to the open-shell analogues without asymmetric charge distributions. These results indicate that the helical systems with intermediate diradical character are potential candidates for the building block of highly...

Braiding of edge states in narrow zigzag graphene nanoribbons: Effects of third-neighbor hopping on transport and magnetic properties

We study narrow zigzag graphene nanoribbons (ZGNRs), employing density functional theory (DFT) simulations and the tight-binding (TB) method. The main result of these calculations is the braiding of the conduction and valence bands, generating Dirac cones for non-commensurate wave vectors $\vec{k}$. Employing a TB Hamiltonian, we show that the braiding is generated by the third-neighbor hopping (N3). We calculate the band structure, the density of states and the conductance, new conductance channels are opened, and the conductance at the Fermi energy assumes integer multiples of the quantum conductance unit $G_{o} = 2e^{2}/h$. We also investigate the satisfaction of the Stoner criterion by these ZGNRs. We calculate the magnetic properties of the fundamental state employing LSDA (spin-unrestricted DFT) and we confirm that ZGNRs with $N=(2,3)$ do not satisfy the Stoner criterion and as such the magnetic order could not be developed at their edges. These results are confirmed by both tight-binding and LSDA calculations.

Transition State Search Using rPM6: Iron- and Manganese-Catalyzed Oxidation Reactions as a Test Case

Abstract We present a reparameterization of PM6 (rPM6) for Fe in a similar procedure to that for Mn in our previous work [T. Saito, Y. Takano, Chem. Lett. 2017, 46, 1567]. The performance of the spin unrestricted rPM6 (UrPM6) method has been examined on both energy minimization and transition state (TS) search on the basis of the spin unrestricted density functional theory (UDFT) calculations. For energy minimization, the UrPM6 outperforms the original UPM6 on the basis of the UB3LYP/SVP-optimized geometries. The test set for TS search is comprised of 58 TS structures for oxidation reactions catalyzed by a variety of iron and manganese complexes including active sites of cytochrome P450 and soluble methane monooxygenase as well as their biomimetic complexes. The standard UPM6 method can locate only 21 out of 58 TS structures (36%), whereas an overall success rate is 83% (48 out of 58) by means of UrPM6. Furthermore, the UrPM6-optimized structures can be obtained easily (on the order of several minutes) and 94% of those are found to be utilized for the subsequent refinement by UDFT without any modifications of geometrical parameters.

Theoretical Study on Third-Order Nonlinear Optical Property of One-Dimensional Cyclic Thiazyl Radical Aggregates: Intermolecular Distance, Open-Shell Nature, and Spin State Dependences

Using the spin-unrestricted density functional theory method, we investigate the relationship between structure, spin state, and second hyperpolarizability (γ) of the finite and infinite one-dimensional open-shell aggregates composed of cyclic thiazyl radicals, that is, 1,2,3,5-dithiadiazolyl (DTDA) radicals. The DTDA aggregates with antiferromagnetic spin-alignment exhibit much greater enhancement of γ than the aggregates with ferromagnetic spin-alignment and the closed-shell benzene aggregates due to the intermediate open-shell singlet nature of the antiferromagnetic DTDA aggregate. It is found that this enhancement shows strong intermolecular distance dependences: the intermolecular distance giving the largest enhancement of γ decreases with increasing the number of molecules, and intermolecular distance alternation reduces the γ. For the infinite antiferromagnetic DTDA aggregate with a realistic intermolecular distance d = 3.1 A, the γ per monomer reaches 1.9 × 106 au, which is comparable to that of t...

RPM6 parameters for phosphorous and sulphur-containing open-shell molecules

ABSTRACTIn this article, we have introduced a reparameterisation of PM6 (rPM6) for phosphorus and sulphur to achieve a better description of open-shell species containing the two elements. Two sets of the parameters have been optimised separately using our training sets. The performance of the spin-unrestricted rPM6 (UrPM6) method with the optimised parameters is evaluated against 14 radical species, which contain either phosphorus or sulphur atom, comparing with the original UPM6 and the spin-unrestricted density functional theory (UDFT) methods. The standard UPM6 calculations fail to describe the adiabatic singlet-triplet energy gaps correctly, and may cause significant structural mismatches with UDFT-optimised geometries. Leaving aside three difficult cases, tests on 11 open-shell molecules strongly indicate the superior performance of UrPM6, which provides much better agreement with the results of UDFT methods for geometric and electronic properties.

Study on adsorption and diffusion of lithium on nitrogen doped silicon carbide nanotubes by density functional theory

Spin-unrestricted density functional theory (DFT) calculation was carried out to investigate the adsorption and diffusion of lithium atom on pristine and N-doped silicon carbides (SiCNTs). It is shown that N-doping reduced the structural stability of SiCNTs and narrowed the HOMO-LUMO gaps. Moreover, N-doping enhanced the lithium storage capacity of SiCNT anode material to some extent and lowered the energy barriers.

Theoretical Study on the Open-Shell Singlet Nature and the Second Hyperpolarizabilities of Corannulene Derivatives with Two Phenoxyl Radicals.

Using the spin-unrestricted density functional theory method, we investigate the interplay between the diradical character y and second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale) of corannulene derivatives with two phenoxyl radicals. This molecule in the singlet state exhibits intermediate y and thus displays a significantly larger γ value than the triplet state and the closed-shell bis-phenol analogue. We also examine the planar molecules involving a coronene moiety in place of the curved corannulene. The intermediate y and large γ values of the corannulene systems are found to originate not from their curved skeleton but from the equilibrium between benzenoid/quinoid resonance forms due to delocalization of the radical electrons of the terminal phenoxyl rings. The longitudinal γ value of the singlet state is found to be comparable to that of s-indaceno[1,2,3-cd;5,6,7-c'd']diphenalene, which is known to be one of the organic molecules with the largest two-photon absorption cross section in this size of pure hydrocarbons. The present system is thus expected to be a promising candidate for highly efficient open-shell NLO molecules.

Assessment of semi-empirical molecular orbital calculations for describing magnetic interactions

We report a thorough comparison between the spin-unrestricted semi-empirical molecular orbital (SE-UMO) and density functional theory (UDFT) calculations for a variety of π-conjugated diradical molecules which exhibit through-bond magnetic exchange interactions. The spin-unrestricted neglect of diatomic differential overlap (NDDO)-based methods (UPM6 and UrPM6) and self-consistent charge density functional tight-biding (UDFTB) are examined. The UCAM-B3LYP method is used as a representative of UDFT. We have found that NDDO-based and UDFTB methods give different performances with respect to the results of UCAM-B3LYP. The degree of diradical character in the target systems decreases in the order of UPM6>UrPM6≈UCAM-B3LYP>UDFTB. The conventional UPM6 and UDFTB calculations tend to overestimate and underestimate the diradical character, respectively, whereas UrPM6 is able to provide much the same results as UCAM-B3LYP. This is convincing evidence that UrPM6 will be useful for in silico screening of materials science at much lower computational cost.

DFT Investigation on the Electronic and Water Adsorption Properties of Pristine and N-Doped TiO2 Nanotubes for Photocatalytic Water Splitting Applications

Experimental studies have shown the production of hydrogen through a photocatalytic water splitting process using a titanium dioxide nanotube (TiO2NT) as a photoelectrode. In this study, a theoretical model of pristine and nitrogen-doped TiO2NT based on a TiO2 anatase (101) surface is presented. Spin unrestricted density functional theory calculations were performed to provide a detailed description of the geometries, electronic properties, and adsorption of water (H2O) on pristine and N-doped TiO2NT. The calculations show that doping with N will improve the photocatalytic properties of TiO2NT in two ways: First, the energy barrier of the dissociation reaction of water into hydroxyl radical and hydrogen atom is reduced; and second, the defect-induced states above the valence band lowers the band gap which will result in enhanced visible-light-driven photoactivity. Based on the position of the Fermi level relative to the defect induced energy levels, an optimal doping concentration of around 1.4% is proposed, which is in good agreement with experimental results. This study provides an atomic/molecular level understanding of the photocatalytic water splitting process and may serve as a groundwork for the rational design of more efficient photocatalysts.

Hydrogen adsorption on boron nitride nanotubes functionalized with transition metals

The implementation and effectiveness of the adsorption of H2 on Transition Metal-doped boron nitride nanotubes (TM/BNNT) with BN defects was evaluated with the aide of spin-unrestricted Density Functional Theory formalism. The results indicate that due to the formation of non-hexagonal rings in the defective BNNT model arising from the removal of a BN bond, the TM preferentially occupies the nitrogen over the boron site of the defect. Among nanotubes doped with TMs, Titanium and Vanadium exhibits qualities appropriate to act as a media for hydrogen storage. Hybridization of the 3d of both Ti and V orbital with the H-1s orbital contributes to the hydrogen adsorption. Ti and V clustering was suppressed by preferential binding of Ti/V atoms on BNNT defects. Finally, the proposed Ti/BNNT as a hydrogen storage substrate is better than the recently predicted Nitrogen doped Carbon Nanotube with divacancy due to its superior heteropolar binding nature such that each Ti can hold up to seven hydrogen molecules near its vicinity that corresponds to a 7.17 wt% H2 with reversible adsorption energy at room temperature.

Theoretical investigation of the solubilization of COOH-functionalized single wall carbon nanotubes in water

The paper discusses the impact of functionalization in understanding the solubility of (n, 0) single walled carbon nanotube (SWCNT) with varying diameters (n=8–16) to increase its dispersion in water with the aide of spin-unrestricted density functional theory (DFT). A finite nanotube model saturated with hydrogen at both ends was functionalized with a carboxylic acid at the sidewall. Functionalization resulted in an enhancement in the solubility of the nanotubes in water which can be explained by the increase in dipole moment as visualized in the HOMO–LUMO surface plots. This behavior depends on the tube diameter marked with saw tooth like periodic features which originated from their different π bonding structures manifested in the electronic band gaps. Furthermore, as the degree of sidewall functionalization increases, the SWCNT sample becomes more soluble as assessed by the calculated Gibbs free energies of solvation.